1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film using laser light (hereafter referred to as laser annealing), and to a laser apparatus for performing laser annealing (an apparatus containing a laser and an optical system for guiding the laser light output from the laser to a processing piece. In addition, the present invention relates to a semiconductor device formed by using that type of laser annealing method, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
The development of thin film transistors (hereafter referred to as TFTs) has been advancing in recent years, and TFTs using a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film have been in the spotlight. In particular, the TFTs are used as elements forming a driver circuit for controlling a pixel, or an element which switches the pixel, in a liquid crystal display device (liquid crystal display) or an EL (electroluminescence) display device (EL display).
A technique of crystallizing an amorphous silicon film into a polysilicon film is generally used as a means of obtaining the polysilicon film. In particular, recently a method of crystallizing the amorphous silicon film using laser light has been gathering attention. A means of obtaining a crystalline semiconductor film by crystallizing an amorphous semiconductor film using laser light is referred to as laser crystallization throughout this specification.
Instantaneous heat treatment of the semiconductor film is possible with laser crystallization, and laser crystallization is an effective technique as a means of annealing the semiconductor film formed on a substrate having low heat resistance, such as a glass substrate or a plastic substrate. Furthermore, the throughput is remarkably high compared to a heat treatment means using a conventional electric furnace (hereafter referred to as furnace annealing).
There are many types of laser light, but generally laser crystallization which uses laser light having a pulse emission type excimer laser as an emission source (hereafter referred to as excimer laser light) is employed. The excimer laser has the advantages of high output and being capable of repeated irradiation at a high frequency, and in addition, the excimer laser light has the advantage of having a high absorption coefficient with respect to a silicon film.
The problem drawing the most attention at present is how large can the grain size of a crystalline semiconductor film crystallized by laser light be made. Naturally, if one grain becomes large, then especially the number of grain boundaries crossing a channel forming region of a TFT will be reduced. It therefore becomes possible to improve the electric field effect mobility and the threshold voltage of the TFT, typical electrical characteristics.
Furthermore, relatively clean crystallinity is maintained within each grain, and in order to increase the TFT characteristics as stated above, it is preferable to form the TFT so as to have the channel forming region completely within one grain.
However, it is difficult to obtain a crystalline semiconductor film with a sufficiently large grain size by present techniques, and although there are reports of such films being obtained experimentally, at present this has not reached a level which can be put to practical use.
Experimental results such as those shown in Shimizu, K., Sugiura, O., and Matsumura, M., “High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, IEEE Transactions on Electron Devices, Vol. 40, No. 1, pp. 112–7, 1993, have been obtained. A three layer structure of Si/SiO2/n+Si is formed on a substrate in the above publication, and is then irradiated by excimer laser light on both the Si layer side and the n+Si layer side. It is shown that a large grain size can be achieved by this type of structure.